The present invention relates to fiber optic cables, and more particularly, to fiber optic cables for use in harsh environments
With advancements in the area of fiber optic sensors, particularly for use in harsh environments, such as in oil and gas wells, there is an increasing need for fiber optic cables that can survive harsh environments. For example, the harsh environment encountered in down-hole fiber optic sensing applications places demanding requirements on the design of fiber optical cables for use in the down-hole environment. Such a fiber optic cable may be used to interconnect a down-hole fiber optic sensor with instrumentation located at the surface of a well bore.
Down-hole environmental conditions can include temperatures in excess of 130xc2x0 C., hydrostatic pressures in excess of 1000 bar, vibration, corrosive chemistry and the presence of high partial pressures of hydrogen. Down-hole applications also lead to the requirement that the fiber optic cable be produced in lengths of 1000 m and longer. Because of the long cable lengths in such applications, the fiber optic cable must be designed to support the optical fiber contained therein from excessive strain associated with the weight of the long length of optical fiber.
The deleterious effects of hydrogen on the optical performance of optical fiber, particularly in sub-sea installations for the telecommunications industry, have long been documented. To protect optical fibers from the effects of hydrogen, hermetic coatings and barriers, such as carbon coatings and the like, have been used to minimize the effects of hydrogen in such sub-sea telecommunications applications. However, at the elevated temperatures experienced in a harsh down-hole environment, such coatings lose their resistance to permeability by hydrogen. Additionally, at such high temperatures, the effects of hydrogen on an optical fiber may be accelerated and enhanced.
Therefore, there exists the need for a fiber optic cable that is suitable for use in such harsh environments.
An object of the present invention is to provide a fiber optic cable for use in a harsh environment.
A further object of the invention is to provide such a fiber optic cable that minimizes the exposure of optical fibers to hydrogen contained in the harsh environment, particularly at high temperatures.
A still further object of the invention is to provide such a fiber optic cable wherein the optical fibers contained in the cable are not exposed to significant damaging strain over a wide range of operating temperatures.
According to the present invention, a fiber optic cable includes a core and a surrounding protective layer. The core includes an inner tube having one or more optical fibers contained therein, and the surrounding protective layer includes an outer tube received over the inner tube, and a layer of buffer material positioned between the outer tube and the inner tube, the buffer material maintaining the inner tube generally centrally located within the outer tube and providing a mechanical link between the inner tube and the outer tube to prevent relative movement therebetween.
According further to the present invention, the inner tube may be coated with a low hydrogen permeability material to minimize the entrance of hydrogen into the inner tube. According still further to the invention, the low hydrogen permeability material may be coated with a protective layer of hard, scratch resistant material to protect the integrity of the low hydrogen permeability material.
In still further accord with the invention, the area in the inner tube may be filled with a filler material, the filler material being selected to have a sufficient viscosity to resist the shear forces applied to it as a result of the weight of the optical fibers within the tube while allowing movement of the optical fibers within the tube during spooling, deployment and handling of the cable to thereby prevent damage and microbending of the optical fibers. According still further to the present invention, the filling material may be impregnated with a hydrogen absorbing/scavenging material.
According further to the invention, the optical fibers have an excess length with respect to the inner tube. According further to the invention, the cable may include an outer jacket of a high temperature, protective material to protect the cable during handling and installation.
The fiber optic cable of the present invention provides a significant advantage over the prior art. The cable provides significant resistance to the damaging effects of hydrogen on an optical fiber by minimizing the exposure of the optical fibers to hydrogen. The inner tube of the cable is coated with a low hydrogen permeability material to limit the ingress of hydrogen into the inner tube. Additionally, the filling material within the inner tube is impregnated with a hydrogen absorbing/scavenging material to remove any hydrogen that may enter the inner tube. A protective coating is received over the low hydrogen permeability material to maintain the integrity of the coating for handling and manufacturing of the cable. To provide a high strength cable capable of deployment in a harsh environment, the inner tube is surrounded by a protective layer that includes a buffer material surrounded by an outer tube.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings.